1. Field of the Invention
The present invention relates to a solid-state imaging device, a method for manufacturing a solid-state imaging device, and an electronic device.
2. Description of the Related Art
Electronic devices such as a digital video camera and a digital still camera include a solid-state imaging device. Such electronic devices include a complementary metal oxide semiconductor (CMOS) type image sensor or a charge coupled device (CCD) type image sensor, for example, as the solid-state imaging device.
In a solid-state imaging device, a plurality of pixels are arranged in matrix on an imaging surface of a semiconductor substrate. Each of the plurality of pixels is provided with a photoelectric conversion portion. A photodiode, for example, is provided as the photoelectric conversion portion.
Among solid-state imaging devices, a CMOS type image sensor has a pixel which is provided with a semiconductor element such as a pixel transistor as well as the photoelectric conversion portion. As the pixel transistor, a plurality of transistors are provided to read out a signal charge produced by the photoelectric conversion portion and output the signal charge as an electric signal to a signal line.
In a solid-state imaging device, the photoelectric conversion portion receives light which is incident through an optical component such as a color filter, at a light receiving surface and photoelectrically converts the light so as to produce a signal charge.
For example, when color filters of three primary colors of red, green, and blue are arranged along an imaging surface in a manner to correspond to each pixel, the photoelectric conversion portion receives colored light which is colored by each of the color filters of the respective colors, in each pixel. That is, focusing on a pixel above which a red color filter is provided, the red color filter absorbs light of green and blue components in light which is incident as an object image and transmits only light of a red component therethrough, and the photoelectric conversion portion receives the transmitted light of the red component below the red color filter.
The solid-state imaging device is demanded to be small-sized, and at the same time, to have increased number of pixels. Accordingly, a size of one pixel is made smaller, so that the pixel has more difficulty in receiving sufficient amount of light. Thus, it is not easy to improve image quality of a picked-up image.
In order to eliminate such defect, a “stacked photoelectric conversion layer type” is proposed. In the “stacked photoelectric conversion layer type”, photoelectric conversion portions of respective colors are not arranged in a direction along an imaging surface, but the photoelectric conversion portions which selectively receive light of respective colors are layered in a vertical direction with respect to the imaging surface. For example, three layers of photoelectric conversion films made of an organic material are stacked so as to sequentially receive light of each of the three primary colors included in incident light (refer to Japanese Unexamined Patent Application Publication No. 2004-335626, Japanese Unexamined Patent Application Publication No. 2005-347356, Japanese Unexamined Patent Application Publication No. 2005-353626, Japanese Unexamined Patent Application Publication No. 2003-234460, and “CMOS Image Sensor with Stacked Organic Photoelectric Conversion Layers” by M. IHAMA et al., FUJIFILM RESEARCH & DEVELOPMENT (No. 52-2007), p. 3-6, for example).
In the “stacked photoelectric conversion layer type”, part of incident light is absorbed at a photoelectric conversion film which is provided at an upper side and is photoelectrically converted. Then, other part of the light which is not absorbed at the photoelectric conversion film provided at the upper side is absorbed by another photoelectric conversion film positioned below the above-mentioned film and is photoelectrically converted. For example, an uppermost photoelectric conversion film absorbs blue color light, an intermediate photoelectric conversion film absorbs green color light, and a lowermost photoelectric conversion film absorbs red color light. Then, photoelectric conversion is performed in each of the photoelectric conversion films so as to produce a signal charge.
Thus, in the “stacked photoelectric conversion layer type”, not merely colored light of a single color but colored light of a plurality of colors is received at each pixel position. Accordingly, light utilization efficiency can be improved, whereby small-sizing can be easily realized. Specifically, a solid-state imaging device of the “stacked photoelectric conversion layer type” has light utilization efficiency of three times as much as a case where color filters of the three primary colors of red, green, and blue are arranged along an imaging surface, so that an area per pixel can be reduced to be one-third.
In a CMOS type image sensor of the “stacked photoelectric conversion layer type”, semiconductor elements such as a transistor are formed in an integrated manner on a surface of a substrate so as to read out a signal charge produced in photoelectric conversion films which receive respective colored light.
Thus, transistors for a plurality of colors (for example, three primary colors) are formed to be arranged on the surface of the substrate, increasing an area occupied by these transistors. Accordingly, it may be difficult to realize small-sizing.
In order to eliminate such defect, it is proposed to provide a readout circuit for reading out an electric charge immediately beneath a photoelectric conversion film of each color without providing transistors for a plurality of colors in an integrated manner on a surface of a substrate (refer to “The latest trend in CMOS image sensors” by J. OHTA et al., CMC Publishing Co., Ltd, April 2007, p. 119, for example).